The invention concerns a method of frequency synthesis in a transmitter transmitting signal bursts to a TDMA communication system, said frequency synthesis is based on a phase-locked loop, wherein a data-modulated output signal is generated by a voltage controlled oscillator in the phase-locked loop in response to a modulating bit flow received on an input of the oscillator. Finally, the invention concerns a frequency synthesizing circuit as well as a radio telephone having such a circuit.
When data modulating the VCO in a PLL are constant (zeros or ones) for an period or possess a preponderance of either zeros or ones, the output frequency of the VCO differs from the nominal centre frequency in this, period owing to the modulation with data. The PLL therefore reacts by trying to bring the frequency back to the nominal centre frequency. This is a problem in particular in connection with the telephone which is associated with digital cordless systems in which the VCO is modulated directly with data.
To obtain rapid locking, the loop must have a high loop bandwidth (about 20 kHz), while a low loop bandwidth (about 2 kHz) is required to prevent the modulation frequency from affecting the PLL. If the modulation in a data packet does not vary or possess a preponderance of either zeros or ones, the modulation frequency will be within the loop bandwidth.
Some therefore work with a concept wherein the loop is closed when searching on a frequency, and is opened during modulation with data. This has several drawbacks, one of which being that the capacitors of the loop are discharged, resulting in frequency drift. Finally, the transmitter power is increased, resulting in battery voltage drop and change in load. This is unfortunate, as the VCO is extremely sensitive to variations in the battery voltage and to changes in the load. The transmitter leaks output power to the VCO, which results in frequency jumps because of the open loop. When the loop is opened, modulation occurs because of mechanical vibrations, and VCO phase noise is not reduced within the loop bandwidth. While, with a view to the long uniform bit trains, it appears to be expedient to open the loop, this involves a number of considerable problems.